A condenser microphone includes a microphone unit in which a diaphragm and a backplate are arranged so as to be opposed to each other. The microphone unit incorporates an impedance converter because of its very high impedance. As the impedance converter, a field effect transistor (FET) is usually used, and on rare occasions, a vacuum tube is used.
An electronic circuit for audio output of the condenser microphone is housed in a metallic microphone case in a state of being mounted on a substrate. Usually, at one end of the microphone case, a three-pin type output connector specified in EIAJ RC5236 (Audio latch lock round type connector) is mounted. The output connector is connected to a phantom power source via an output cable (balanced shielded cable).
If strong electromagnetic waves radiated from a cellular phone or the like are applied to the microphone or the output cable, the electromagnetic waves pass through the output cable and intrude into the microphone via the output connector. In the microphone, the electromagnetic waves are sometimes demodulated by the impedance converter and delivered from the microphone as noise having an audio frequency.
To prevent this phenomenon, No. 1 pin for grounding of the three pins that the output connector has is connected to the microphone case, and an outer ring of a male plug on the output cable side, which connects with a shield coating, is brought into contact with the inner surface of the microphone case to provide electrical connection, by which a shielding function is given to the microphone case.
A high frequency current flowing in the microphone case due to the electromagnetic waves flows on the surface only due to the skin effect. Usually, the microphone case is manufactured by casting (die casting) of zinc, aluminum, etc. from the viewpoint of workability of microphone case and cost constraints, and the resistivity of casting surface is increased by an oxide film as compared with the base material, so that a sufficient shielding effect cannot be achieved.
The resistivity of the surface can be decreased to some extent by removing the oxide film on the casting surface by cutting. However, usually, since a casting has blowholes therein due to a gas at the time of casting operation, even if the surface is cut, there is no guarantee that the resistivity of the surface necessarily decrease. Also, in the case where painting is performed, the resistivity of the surface is increased by the surface treatment (chromate treatment etc.).